Particulate metal alloy coating for providing corrosion protection

ABSTRACT

Coatings containing particulate metal alloy are disclosed. The coatings provide corrosion protection to a substrate, such as a metal substrate. The coatings contain zinc-metal-containing alloy in flake form, most particularly an alloy flake of zinc and aluminum. The coating can be from compositions that are water-based or solvent-based. The compositions for providing the coating may also contain a substituent such as a water-reducible organofunctional silane, or a hexavalent-chromium-providing substance, or a titanate polymer, or a silica substance constituent. The coating may desirably be topcoated.

CROSS-REFERENCE TO RELATED APPLICATION

[0001] This application claims the benefit of U.S. ProvisionalApplication No. 60/268,273, filed Feb. 14, 2001.

BACKGROUND OF THE INVENTION

[0002] A variety of coating compositions having a liquid medium,generally at least substantially resin-free, which can bechromium-containing coating compositions, but that can also includethose that don't contain chromium, and all typically for protectingferrous substrates, are known. Of special interest are those whichcontain particulate metal. Representative coating compositions of thistype that were initially developed could be quite simplistic, such ascompositions containing essentially chromic acid and particulate metalin an alcohol medium, as disclosed in the U.S. Pat. No. 3,687,738.

[0003] A later development of particular effectiveness for providing acorrosion-resistant coating on metal substrates was the more complexcomposition such as shown in U.S. Pat. No. 3,907,608. The compositioncomprised chromic acid, or equivalent, a particulate metal of mainlyzinc or aluminum, wetter and a liquid medium comprising water plushigh-boiling organic liquid. The composition had very desirable coatingcharacteristics when including a viscosity modifier such as awater-soluble cellulose ether, as disclosed in U.S. Pat. No. 3,940,280.

[0004] The coating could be especially useful as an undercoating. Thusit has been taught to use such a more complex coating composition as anundercoating over ferrous surfaces. The coating is then provided with asilicate topcoating, as disclosed in U.S. Pat. No. 4,365,003.

[0005] It has been known that where coating compositions could containthe particulate metal as untreated aluminum flake, such flake can beunstable in water-based coating compositions. In such water-basedcoating compositions, standard aluminum flake will react with water inthe composition to form hydrogen gas. One approach for avoiding thisproblem has been to coat the aluminum flake. One such coating is anacrylic coating formed by reacting mono-ethylenically unsaturated silanewith acrylic monomers having amine hydroxyl or epoxy groups, asdisclosed in U.S. Pat. No. 4,213,886. However, these products arespecialty items tailored to provide a coating of good glamour appearanceand have not found a wide acceptance.

[0006] There has also been proposed the preparation of coatingcompositions that contain hydrolyzed organotrihydrocarbonoxy silane anda particulate metal. These compositions, such as disclosed in U.S. Pat.No. 4,218,354, can provide corrosion protection to a coated substrate.The silanes utilized are not water-reducible and, thus, it can beexpected that the compositions are best formulated in the presence oforganic liquid.

[0007] More recently, it has been taught in U.S. Pat. No. 5,868,819 thatcomposition substituents which are epoxy functional silanes, and whichare water-reducible, can be useful in forming compositions for coatingmetal substrates. The compositions rely on a variety of ingredients toprovide for a chrome-free system.

[0008] Other compositions containing particulate metal and findingutility by providing corrosion protection for a substrate are wellknown. Some of these will be more particularly discussed hereinbelow. Itwould be desirable to provide a coating from all such compositions, andalso to provide a coating combination of undercoating plus topcoating,each of which could have wide acceptance. It would further be desirableto provide same, which would offer outstanding corrosion protection tometal substrates and be efficiently and economically produced.

SUMMARY OF THE INVENTION

[0009] The present invention can offer such features. The coating offersoutstanding corrosion-resistance such as on coated steel parts. Inaddition to corrosion-resistance, deposited films can have excellentcoating adhesion. Coating compositions for the combination may beone-package compositions, and in such case provide ease of preparation,storage and transport as well as use. Coating compositions that aretypically one-package compositions may lend themselves to extendedstorage stability.

[0010] In one aspect, the invention is directed to a coating compositionadapted for application to, and curing on, a substrate, whichcomposition contains particulate metal in a liquid medium and providescorrosion resistance as a cured coating on the substrate, wherein thereis provided the improvement in the particulate metal constituency ofsuch composition comprising:

[0011] zinc alloy in flake form comprising greater than 50 weightpercent zinc in the alloy flake and a balance that is less than 50weight percent of non-zinc alloy metal in the alloy flake. In anotheraspect, the invention is directed to preparing a corrosion-resistantcoated substrate in a method utilizing this coating composition andcuring applied coating composition on a substrate.

[0012] In another aspect, the invention is directed to a coatedsubstrate protected with a chrome-free, corrosion-resistant coating froma composition comprising:

[0013] (A) liquid medium;

[0014] (B) zinc alloy in flake form comprising greater than 50 weightpercent zinc in said alloy flake, and a balance of less than 50 weightpercent of additional alloy metal; and

[0015] (C) silane binding agent.

[0016] In another aspect, the invention includes the method of preparinga corrosion-resistant coated substrate protected with a chrome-free,corrosion-resistant coating, which method comprises:

[0017] (1) applying to the substrate a coating composition comprising:

[0018] (A) liquid medium;

[0019] (B) zinc alloy in flake form comprising greater than 50 weightpercent zinc in the alloy flake, and a balance of less than 50 weightpercent of additional alloy metal; and

[0020] (C) silane binding agent; with the coating composition beingapplied in an amount sufficient to provide, upon curing, above about 500but not substantially above about 9,000 mg/ft² of coating on the coatedsubstrate; and

[0021] (2) heat curing applied coating composition on the substrate at atemperature up to about 700° F. for a time of at least about 10 minutes.

[0022] In another aspect, the invention is directed to a coatedsubstrate protected with a corrosion-resistant coating from a coatingcomposition comprising:

[0023] (A) liquid medium;

[0024] (B) zinc alloy in flake form comprising greater than 50 weightpercent zinc in said alloy flake and a balance of less than 50 weightpercent on non-zinc alloy metal; and

[0025] (C) a hexavalent-chromium-providing substance.

[0026] An aspect of the invention also includes preparing acorrosion-resistant coated substrate utilizing this coating andemploying coating amounts and curing conditions as describedhereinabove.

[0027] In yet another aspect, the invention is directed to a coatedsubstrate protected with a corrosion-resistant coating from the coatingcomposition comprising:

[0028] (A) zinc alloy in flake form comprising greater than 50 weightpercent zinc in said alloy flake and a balance of less than 50 weightpercent on non-zinc alloy metal;

[0029] (B) a titanate polymer; and

[0030] (C) a liquid vehicle comprising organic liquid for such titanatepolymer.

[0031] The method of preparing a corrosion-resistant coated substrateutilizing this coating composition is a further invention aspect,particularly to curing applied coating at a temperature up to about 600°F. for a time of at least about 10 minutes.

[0032] In a still further aspect, the invention is directed to a coatedsubstrate protected with a corrosion-resistant coating from the coatingcomposition comprising:

[0033] (A) liquid medium;

[0034] (B) zinc alloy in flake form comprising greater than 50 weightpercent zinc in said alloy flake and a balance of less than 50 weightpercent on non-zinc alloy metal; and

[0035] (C) one or more of a water-soluble and water dispersible silicasubstance.

[0036] Considerations for preparing a corrosion-resistant coatedsubstrate with the coating composition also apply as an inventionaspect, particularly to curing applied coating composition up to about700° F. for a time of at least about 10 minutes.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0037] The particulate metal of the coating composition will be a zincalloy in flake form. The particulate metal is an alloy of zinc in flakeform generally comprising at least greater than 50 weight percent zinc,usually depending on the particular alloy. The balance of the alloy isthus less than 50 weight percent of non-zinc alloy metal. For azinc/aluminum alloy the zinc advantageously comprises greater than about80 weight percent zinc, metals basis, or, conversely, less than about 20weight percent aluminum, and preferably greater than about 85 weightpercent zinc, or less than about 15 weight percent aluminum. For azinc/tin alloy, on a metals basis, there is advantageously at leastabout 70 weight percent of zinc or, conversely, not more than about 30weight percent tin. For considering corrosion resistance of coatedsubstrate, the alloy is preferably an alloy of zinc and aluminum inflake form.

[0038] Advantageously for economy of alloy production, the zinc alloywill be in flake form in a paste. In the alloy of zinc with aluminum ina paste, the aluminum may most advantageously be present in an amount upto about 8 weight percent, basis weight of the paste. For corrosionresistance of the coating balanced with coating adhesion, the aluminumwill preferably not contribute more than about 5 weight percent, basisweight of the paste. The total of the zinc alloy flake will generallycontribute from 90 to 95 weight percent or more of the paste, with abalance of paste liquid. The alloy flake paste can contribute someliquid in minor amount, e.g., dipropylene glycol or mineral spirits, orsome liquid even in trace amount to the coating composition. It has beenfound that the zinc alloy flake paste can be generally useful, togetherwith other coating composition ingredients, for those coatingcompositions that will contain particulate metal in paste form. It isalso contemplated that the special alloy flake can be useful whenprepared in dry form. Such dry form paste can thus be 99 weight percentor more of alloy flake.

[0039] Zinc alloys in flake form other than the zinc and aluminum alloy,or zinc and tin alloy, can be useful. These include zinc alloyed withone or more of magnesium, nickel, cobalt and manganese. A representativethree-metal alloy flake is zinc-aluminum-magnesium alloy. The preferredalloy paste for the undercoating composition is STAPA 4ZnAl7 made byEckart-Werke, which is a paste of zinc and aluminum alloy in flake formtypically containing from about 85 to about 86 weight percent zinc, fromabout 4 to about 8 weight percent aluminum and a balance of from about 7to about 10 weight percent paste liquid, all basis 100 weight percent ofthe paste. Such a paste containing from about 4 to about 5 weightpercent of aluminum in the alloy is also of particular interest. Withregard to particle size, the preferred alloy flake of zinc and aluminumwill generally have a size such that at least about 50 weight percent ofthe flake particles will have a longest dimension of less than about 13microns, and preferably has at least about 90 weight percent of theparticles with a longest dimension of less than about 15 microns. Forconvenience, the non-alloy balance of the paste, i.e., the about 7 toabout 10 weight percent balance, is referred to herein for convenienceas the paste liquid. It will, however, be understood that this balancemay contain substituents, e.g., lubricants used in making the paste,that are not in liquid form when the paste is utilized in the coatingcomposition.

[0040] It is to be understood that the zinc alloy flake may be presentin a coating composition with other pulverulent metal, which is innon-flake form, e.g., zinc, aluminum, aluminum alloy, or mixturesthereof in pulverulent, non-flake form. Thus, this pulverulent metal innon-flake form may also be in non-alloy form. Such metal in other formmay be incorporated with the paste, but more typically will be blendedinto the coating composition generally, although it could be used inboth. Typically only minor amounts of such other pulverulent metal, innon-flake form, will be present in the coating composition, and theincorporation of such other metal is often avoided. Usually, thenon-flake metal might be in powder form.

[0041] Although it is contemplated that the zinc alloy flake will beuseful in any coating composition wherein particulate metal is utilizedin a liquid vehicle (or “liquid medium” as such term is used herein) toprepare a corrosion-resistant coating, several compositions are ofparticular interest. Hereinbelow, these coating compositions ofparticular interest will next be discussed.

A. CHROMIUM-FREE COATING COMPOSITION

[0042] These coating compositions, when prepared in final form forapplication to a substrate, will usually be referred to simply as the“coating composition”. These are compositions that contain a silanesubstituent, as will be more particularly described hereinbelow.Preferred coating compositions of this type have been described in U.S.Pat. No. 5,868,819. Generally, these preferred compositions may bereferred to as a “water-reducible coating composition”. For supplyingthe liquid medium of the coating composition, there will usually be usedwater in combination with organic liquid, which organic liquid may alsobe referred to herein as the “solvent”. Some of these chromium-freecoating compositions will be merely solvent based. Those that containwater in general will be infinitely dilutable with water.

[0043] Water is present in the preferred compositions in an amount fromat least about 20, and generally not above about 70 weight percent,basis total composition weight. The organic liquid of these preferred,water-reducible compositions can be a low-boiling organic liquid, suchas disclosed in U.S. Pat. No. 6,270,884, although it also can be ahigh-boiling organic liquid, and may include mixtures of the foregoing.High-boiling organic liquids that boil above about 100° C. have beendisclosed in U.S. Pat. No. 5,868,819. The low-boiling organic liquidshave a boiling point at atmospheric pressure below about 100° C., andare preferably water-soluble. Such may be represented by acetone, or lowmolecular weight alcohols such as methanol, ethanol, n-propylalcohol andisopropylalcohol, and further include ketones that boil below 100° C.,such as waters soluble ketones, e.g., methyl ethyl ketone.

[0044] Generally, the organic liquid will be present in the preferredwater-reducible compositions in an amount from about 1 to about 30weight percent, basis total composition weight. Preferably, for economyplus ease of composition preparation, acetone will supply thelow-boiling organic liquid and will be present in an amount betweenabout 1 and about 10 weight percent of the total composition. Generally,representative high-boiling organic liquids for the preferredwater-reducible compositions contain carbon, oxygen and hydrogen. Theymay have at least one oxygen-containing constituent that may behydroxyl, or oxo, or a low molecular weight ether group, i.e., a C₁-C₄ether group. Since water dispersibility and preferably water solubilityis sought, high molecular weight polymeric hydrocarbons are notparticularly suitable, and advantageously serviceable hydrocarbonscontain less than about 15 carbon atoms and have a molecular weight of400 or less. Particular hydrocarbons, which may be present ashigh-boiling organic liquid, include tri- and tetraethylene glycol, di-and tripropylene glycol, the monomethyl, dimethyl, and ethyl ethers ofthese glycols, low molecular weight liquid polypropylene glycols, aswell as diacetone alcohol, the low molecular weight ethers of diethyleneglycol, and mixtures of the foregoing. Where the organic liquid is amixture of high-boiling organic liquid with low-boiling organic liquid,such a mixture can be represented by acetone plus dipropylene glycol.

[0045] In addition to the particulate metal and the liquid medium,another necessary ingredient in these compositions is silane, sometimesreferred to herein as the “silane binding agent”. For the preferredcompositions, these can include water-reducible, organofunctionalsilane. To be water-reducible, the silane should be readily dilutablewith water and is preferably completely dilutable with water. The usefulsilane for other than the preferred compositions can be one where thesilane must have a cosolvent present when reduced with water, so as toprevent gellation on water reduction, or to prevent the formation of aprecipitate. For example, silanes such as the organotrihydrocarbonoxysilanes of U.S. Pat. No. 4,218,354, and as represented bymethyltriethoxy silane, are useful for other than the preferredwater-reducible compositions. These silanes must be blended with acosolvent and water, e.g., ethylene glycol monoethyl ether and water.For these silanes, the silane and water react such that without thecosolvent, fast gellation would be typical. In this regard, the silaneswhich are useful herein in the preferred chromium-free compositions arenon-gelling, water-reducible silanes. In these silanes, theorganofunctionality can be represented by vinyl, e.g., as invinyltrimethoxysilane, or methacryloxy, such as inmethacryloxypropyl-trimethoxysilane, and amino, as in3-amino-propyltrimethoxysilane, but is preferably epoxy functional forenhanced coating performance as well as composition stability. Thesegenerally contain the —Si(OCH₃)₃ functionality, or the —Si(OCH₂CH₃)₃ or—Si(OCH₂CH₂CH₃)₃ functionality. These silanes will generally contributefrom about 3 weight percent to about 20 weight percent of the totalcomposition weight. Preferably, the useful silane in the preferredwater-reducible composition is an epoxy functional silane such asbeta-(3,4-epoxycyclohexyl) ethyltrimethoxy-silane, 4(trimethoxysilyl)butane-1,2 epoxide or gamma-glycidoxypropyltrimethoxysilane.

[0046] For the purpose of helping the dispersing of the zinc alloy flakein the chromium-free coating composition, there may be added adispersing agent, i.e., surfactant, serving as a “wetting agent” or“wetter”, as such terms are used herein. Suitable such wetting agents ormixture of wetting agents can include nonionic agents such as thenonionic alkylphenol polyethoxy adducts, for example. Also, there can beused anionic wetting agents, and these are most advantageouslycontrolled foam anionic wetting agents. Serviceable such wetting agentsor mixture of wetting agents can include anionic agents such as organicphosphate esters, as well as the diester sulfosuccinates as representedby sodium bistridecyl sulfosuccinate. The amount of such wetting agentis typically present in an amount from about 0.01 to about 10 weightpercent of the total coating composition.

[0047] These coating compositions, in general, may also containadditional ingredients. As additional ingredients, the coatingcomposition may also contain what is usually referred to herein as a“boric acid component”, or “boron-containing compound”. It is convenientto use orthoboric acid, commercially available as “boric acid”, althoughit is also possible to use various products obtained by heating anddehydrating orthoboric acid, such as metaboric acid, tetraboric acid andboron oxide, as well as salts, e.g., zinc borate or the like. The boricacid component should be present in an amount of at least about 0.1weight percent and may be present in an amount up to about 10 weightpercent or more of the preferred composition.

[0048] The compositions may also contain a pH modifier, which is able toadjust the pH of the composition. The pH modifier is generally selectedfrom the oxides and hydroxides of alkali metals, with lithium and sodiumas the preferred alkali metals for enhanced coating integrity; or, it isselected from the oxides and hydroxides usually of the metals belongingto the Groups IIA and IIB in the Periodic Table, which compounds aresoluble in aqueous solution, such as compounds of strontium, calcium,barium, magnesium, zinc and cadmium. The pH modifier may also be anothercompound, e.g., a carbonate or nitrate, of the foregoing metals.

[0049] Some of these compositions may also contain thickener. Thethickener, when present, can contribute an amount of between about 0.01to about 2.0 weight percent of thickener, basis total compositionweight. This thickener can be a water-soluble cellulose ether, includingthe “Cellosize” (trademark) thickeners. Suitable thickeners include theethers of hydroxyethylcellulose, methylcellulose,methylhydroxypropylcellulose, ethylhydroxyethylcellulose,methylethylcellulose or mixtures of these substances. Other thickeningagents include xanthan gum, associative thickeners, such as the urethaneassociative thickeners and urethane-free nonionic associativethickeners, which are typically opaque, high-boiling liquids, e.g.,boiling above 100° C. Other suitable thickeners include modified clayssuch as highly beneficiated hectorite clay and organically modified andactivated smectite clay, although such is not preferred.

[0050] Some of these compositions may contain ingredients in addition tothose already enumerated hereinabove. These other ingredients mayinclude phosphates. It is to be understood that phosphorous-containingsubstituents, even in slightly soluble or insoluble form, may bepresent, e.g., as a pigment such as ferrophos. The additionalingredients will frequently be substances that can include inorganicsalts, often employed in the metal coating art for imparting somecorrosion-resistance or enhancement in corrosion-resistance. Materialsinclude calcium nitrate, dibasic ammonium phosphate, aluminumtripolyphosphate, calcium sulfonate, 1-nitropropane, lithium carbonate(also useful as a pH modifier), or the like, and, if used, these aremost usually employed in the coating composition in a total combinedamount of from about 0.1 to about 2 weight percent.

[0051] As mentioned hereinabove, these compositions are “chromium-free”.By being chromium-free it is meant that the composition preferablycontains no chromium ion, e.g., as trivalent or hexavalent chromium,including such chromium in ion form as could be contributed by chromicacid or dichromate salts. If any hexavalent chromium is present,advantageously it should not exceed trace amounts, e.g., be present toprovide less than 0.1 milligram of chromium per square foot of coating,for best environmental concerns. It is to be understood that thecomposition may contain chromium in nonsoluble form, as for examplemetallic chromium contributed as part of the zinc alloy flake. Wherecompositions herein have been described as resin-free, such arepreferably resin-free excepting for trace amounts of resin, but such mayinclude minor amounts of resin, such as a few weight percent, e.g., 1 to2 weight percent, of resin. By resin it is meant the generallysynthetic, polymeric resins, which are typically used as binders inpaint systems, but is not meant to include either thickening agent, whenpresent, or to include the silane binding agent.

[0052] The compositions can be formulated in a variety of procedures.For example, as an alternative to directly using the silane bindingagent in a concentrated form, the silane may be utilized as a moredilute premixture of the silane, such as the silane mixed with adiluent, e.g., a diluent selected from the substituents providing thecoating composition liquid medium, such as water, or water plus boricacid component, or water plus organic liquid including acetone. As anadditional example of a composition preparation procedure, a precursormixture might be prepared from the organic liquid, which may be presenttogether with wetting agent, while further including the metal flake.Packaging concepts, as well as formulation considerations for how thecoating composition is prepared, can be taken into consideration whenbringing undercoating composition ingredients together. Even consideringstorage stability, the water-reducible compositions are, however,preferably always a one-package formulation of all coating compositioningredients.

B. CHROMIUM-CONTAINING COATING COMPOSITION

[0053] The chromium-containing coating compositions need not be complexand yet form highly desirable, corrosion-resistant coatings on thesubstrate metal surface after curing at elevated temperature. Some ofthe very simple chromium-containing undercoating compositions, such ashave been taught in U.S. Pat. No. 3,687,738, can merely contain chromicacid and a particulate metal in liquid medium.

[0054] These corrosion-resistant, chromium-containing compositions cancontain chromic acid as the chromium-providing substance or itsequivalent in aqueous medium, for example, chromium trioxide or chromicacid anhydride. But for some compositions, chromium may be supplied, inwhole or in part, by a salt such as ammonium dichromate, or by sodium orpotassium salts, or by substances such as calcium, barium, magnesium,zinc, cadmium, and strontium dichromate. Additionally, for somecompositions, the hexavalent-chromium-providing substance might be amixed chromium compound, i.e., include trivalent chromium compounds.Although some compositions might contain only a small amount, e.g., 5grams per liter of hexavalent chromium, expressed as CrO₃, and maycontain as much as about 100 grams per liter of composition ofhexavalent chromium, expressed as CrO₃, many compositions will typicallycontain between about 20 to 60 grams.

[0055] Substantially all of these coating compositions are simplywater-based, for economy. But for additional or alternative substances,to supply the liquid medium at least for some of these compositions,there have been taught, as in U.S. Pat. No. 3,437,531, blends ofchlorinated hydrocarbons and a tertiary alcohol including tertiary butylalcohol as well as alcohols other than tertiary butyl alcohol. In theselection of the liquid medium, economy will generally be of majorimportance, and thus such medium will most always contain readilycommercially available liquids.

[0056] Particularly preferred chromium-containing coating compositions,for enhanced coating adhesion as well as corrosion resistance, willcontain thickeners, such as water soluble cellulose ethers and will alsocontain high-boiling organic liquid. For economy, these particularcoating compositions preferably contain between about 0.01 to 3 weightpercent of water soluble cellulose ether, such as hydroxyethylcellulose,methylcellulose, methylhydroxypropylcellulose,ethylhydroxyethylcellulose, methylethylcellulose or mixtures of thesesubstances. Although the cellulose ether needs to be water-soluble toaugment thickening for these particular coating compositions, it neednot be soluble in the high-boiling organic liquid, which liquid cancontribute up to 50 volume percent of the coating composition based onthe total volume of liquid in the coating composition. Such organicliquid, when present, also can supply substantially above about 5 volumepercent, and advantageously above about 15 volume percent, both on thesame basis as for the 50 volume percent, of the coating compositionliquid.

[0057] In addition to the chromium-providing substance, the liquidmedium and the zinc alloy flake, some of these chromium-containingcoating compositions that are water-based will nevertheless contain someorganic liquid. The preferred organic liquid has a boiling point atatmospheric pressure above 100° C., while preferably beingwater-soluble. These have been discussed hereinabove in connection withthe chromium-free coating compositions. Representative preferred coatingcompositions have been discussed in U.S. Pat. No. 3,907,608, which ishereby incorporated by reference. For additional substituents that maybe contained in some of these compositions, e.g., wetters, boric acidcomponent, pH modifiers and other ingredients, reference can be madehereinabove to the discussions of these ingredients for thechromium-free coating compositions.

C. TITANATE BINDER COATING COMPOSITION

[0058] To provide a dark black color, some of the titanate bindercoating compositions may contain manganese dioxide. A representativecoating of this type has been disclosed in U.S. Pat. No. 4,544,581. Bothnatural manganese dioxide (MnO₂ B) from refined ore and syntheticallymanufactured manganese dioxide (MnO₂ M) are satisfactory. Syntheticmanganese dioxide has a higher concentration of Mn and MnO₂ and a largerparticle size (97% vs. 76% passing through a 325-mesh screen). Syntheticmanganese dioxide contains about 2 to 3 percent water while naturalmanganese dioxide has no detectable water. It is usually preferable touse only sufficient manganese dioxide to provide a coating having thedesired darkness for a particular application so that it will providegreater corrosion resistance. When manganese dioxide is present, theamount of manganese dioxide in the coating can be from about 20 to about45 percent by weight of the solids in the coating. This amount ofmanganese dioxide can be on the order of equivalent to from about 30 toabout 100 percent by weight of the zinc alloy flake metal.

[0059] In this titanate binder coating composition, the primary bondingmaterial is generally an organic titanate polymer, which ispolyfunctional. When the coating is heated to a temperature in the rangeof about 275 to 450° F., this titanate polymer produces a purelyinorganic titanium dioxide, which bonds the coating to the metalsubstrate. This heating also initiates a hydrolysis reaction, whichenhances and optimizes the adhesion and abrasion resistance of the driedand cured coating. Suitable titanate bonding materials are alkyl estersof tetraisopropyl titanate, tetrabutyl titanate, 2-ethylhexyl titanateand N-butyl titanate polymer. The amount of titanate polymer in thecoating can be from about 6 to about 20 percent by weight of the solidsin the coating.

[0060] Preferably, to improve film integrity and insure adhesion to asubstrate before the primary bonding material is cured, the titanatebinder undercoating also contains a secondary resin. The amount ofsecondary resin can be about 0.5 to 10 percent by weight of dry filmsolids. Suitable secondary resins include ethyl-hydroxyl-ethylcellulose, polyesters, silicones, epoxy resin in the presence ofcaprolactam pyrrolidone and piperidone, conjugated drying oils,unsaturated carboxylic amides and aromatic asphalt resins.

[0061] Preferably, to insure that the titanate binder coating does notgel prior to application to a substrate and that it has suitable flowand wetting characteristics around edges of the substrate, the coatingcontains a thixotropic agent. A suitable thixotropic agent is silanetreated silica dioxide. The amount of this thixotropic agent in thecoating may be about 0.4 to 12 percent by weight of the particulatemetal, which may be all zinc alloy flake metal. The titanate binderundercoating may also contain a silane such as the silanes detailedhereinabove in connection with the water-reducible, chromium-freecoating compositions. A suspension agent may also be used to ensure thatthe alloy flake metal does not settle out of the titanate binder coatingcomposition. A suitable suspension agent is polyethylene. The amount ofpolyethylene used as a suspension agent may be about 0.2 to 5 percent byweight of the particulate metal, which metal can be all zinc alloy flakemetal.

[0062] To ensure that the titanate binder coating composition does notundergo a hydrolysis reaction before the coating is applied to asubstrate, the coating advantageously contains a water scavenger orhygroscopic agent. Inclusion of a hygroscopic agent is particularlydesirable when a synthetic manganese dioxide pigment is used since itcontains 2 to 3 percent water, which, over a period of time, could atleast partially hydrolize, the titanate bonding material. Suitablehygroscopic agents are calcium oxide, silica dioxide, barium oxide, andpotassium chloride. The amount of hygroscopic agent in the coatingcomposition may be 0.2 percent to 12 percent by weight of theparticulate metal, e.g., all zinc alloy flake metal, and preferablyabout 0.4 percent to 6 percent by weight of such metal.

[0063] The vehicle or carrier of the titanate binder coating compositionmay contain both active and inactive solvents. The active solventsdissolve the titanate primary bonding polymers and the inactive solventsdecrease the cost of the vehicle, are excellent thinners of the coatingcomposition, and are believed to modestly improve adhesion and saltspray resistance by modifying and controlling film flow. The vehiclesolvents may consist of about 10 percent to 60 percent by weight ofinactive solvents and the balance preferably of active solvents.

[0064] Suitable active solvents for the titanate polymers are butylalcohol N-butanol (hereinafter N-butanol), 2-ethylhexanol, cellosolveacetate, heptane, methyl ethyl ketone and methyl isobutyl ketone.Suitable inactive solvents include aromatic hydrocarbons such as xylol,xylene, and toluene. Where the solvent is such as a high-boilinghydrocarbon, as described hereinbefore in connection with thechromium-containing undercoatings, e.g., dipropylene glycol, suchsolvent itself may be serviceable for providing most, to all, of thecomposition vehicle and be compatible with the titanate polymer.

[0065] The coating composition contains sufficient vehicle solvents toproduce the viscosity desired for the particular method of applying theliquid coating to a substrate. For application of the coating to asubstrate by dipping, rolling or spraying, the viscosity of thecomposition in a Zahn No. 2 cup is usually in the range of 20 to 150seconds. A coating composition viscosity in this range can usually beobtained when the vehicle solvents by weight are about 0.9 to 1.5 timesthe weight of all the resins in the composition. A process for making atitanate polymer undercoating composition with active plus inactivesolvents is disclosed in the above-mentioned U.S. Pat. No. 4,544,581,the disclosure of which is incorporated herein by reference.

[0066] As will be understood, a titanate binder coating providing ablack color may be a topcoat. The basecoat may be any of a variety ofcoatings, e.g., one or more of a phosphate pretreatment such as of zincphosphate, or a paint basecoat such as a zinc-rich paint, or a titanatebinder coating without black color. General basecoat and topcoatconsideration, which basecoats may include pretreatments, will bediscussed further hereinbelow.

D. SILICA SUBSTANCE COATING COMPOSITION

[0067] A typical coating of this type includes the zinc alloy metalflake and a silica substance constituent, sometimes referred to hereinas a silica substance “binder” such as sodium silicate. Thewater-soluble or water dispersible binder may also more broadly be analkali metal silicate, an organic silicate ester, e.g., ethyl silicate,a colloidal silica sol or the like. Further, organic ammonium silicateshave been disclosed as binders. The use of ethyl silicate or the likehas been disclosed in U.S. Pat. No. 3,469,071 and the utilization of anorganic ammonium silicate has been disclosed in U.S. Pat. No. 3,372,038.The disclosures of these two patents are hereby incorporated byreference. For convenience, the binder can be referred to herein as asilica substance binder and the composition as a silica substancecoating composition. The liquid medium of these coating compositionswill be a water-based liquid medium and may comprise water such asdeionized water or tap water.

[0068] In addition to the zinc alloy metal flake, silica substance as abinder and liquid medium, these coating compositions can containadditional ingredients. The use as an oxidizing agent of red lead or theperoxides of calcium, magnesium and zinc has been disclosed in U.S. Pat.No. 2,944,919. Additionally, a thickening agent, such as a celluloseether or xanthan gum, as well as a gelling agent, is generally useful.It may be better not to try to add the thickening agent directly butrather, to prepare an aqueous suspension of the thickener and then toadd this suspension to the rest of the vehicle or binder. With hydratedmagnesium silicates, for instance, the addition of 0.32 to 0.66 percentbased on the silica present in the binder can be effective to increasethe undercoating adhesion.

[0069] Lead oxide added to the coating composition may increase the potlife of the composition. The coating may also include inorganicextenders, such as zinc oxide, iron oxide, aluminum oxide, and the like,and inorganic pigments such as titanium oxide. The substances iron oxideand zinc oxide may also be useful as pigments. Mica, bentonite, and thelike may be used to increase flexibility in the coating.

[0070] Coatings: General Considerations

[0071] A. Application

[0072] Usually, the silicate coatings are applied by brush application.As mentioned hereinabove, titanate binder coating compositions typicallymay be applied by dipping, rolling or spraying techniques. Generally,the coating compositions may be applied by any of these varioustechniques, such as immersion techniques, including dip drain and dipspin procedures. Where parts and coating compositions are compatiblewith same, the coating compositions can be applied by curtain coating,brush coating or roller coating and including combinations of theforegoing. It is also contemplated to use spray technique as well ascombinations, e.g., spray and spin and spray and brush techniques.Coated articles that are at an elevated temperature may be coated, oftenwithout extensive cooling, by a procedure such as dip spin, dip drain orspray coat.

[0073] B. Substrates and Undercoats

[0074] The protected substrate can be any substrate, e.g., a ceramic orsimilar substrate, but is most particularly a metal substrate such as azinc or iron, e.g., a steel substrate, an important consideration beingthat any such substrate withstand the heat curing conditions for thecoating. By a “zinc” substrate it is meant a substrate of zinc or zincalloy, or a metal such as steel coated with zinc or zinc alloy, as wellas a substrate containing zinc in intermetallic mixture. Likewise, theiron of the substrate can be in alloy or intermetallic mixture form.

[0075] Especially where such are metal substrates, which are mostusually ferrous substrates, these may already be coated, includingpretreatments, e.g., pretreatment by chromate or phosphate treatment,prior to application of the coating. Particularly for some coatings, thesubstrate may be pretreated to have, for example, an iron phosphatecoating in an amount from about 50 to about 100 mg/ft² or a zincphosphate coating in an amount from about 200 to about 2,000 mg/ft².However, a zinc phosphate coating may be avoided where the undercoatingwill be cured at elevated temperature. In general, the substrate mayhave received any undercoating as is contemplated for use, especiallyfor use with the above-described compositions of particular interest.For further undercoating considerations, reference can be madehereinabove, such as in the discussion of the titanate binder coatings.

[0076] C. Curing and Coating Weight

[0077] After application of the coating composition to the substrate, itis preferred for best corrosion-resistance to subsequently heat-cure theapplied coating, excepting for some silica substance coatings whereair-drying can be effective. However, volatile coating substances may beinitially simply evaporated from any of the applied coatings, e.g., bydrying before curing. Cooling after drying may be obviated. Thetemperature for such drying, which may also be referred to as precuring,can be within the range from about 100° F. up to not essentially aboveabout 250° F. Drying times can be on the order of from about 2 to about25 minutes.

[0078] Any elevated temperature curing of an coating composition on asubstrate will often be a hot air oven cure, although other curingprocedures can be used, e.g., infrared baking and induction curing. Thecoating composition can be heat-cured at elevated temperature, e.g., onthe order of about 450° F., but usually greater, oven air temperature.The cure will typically provide a substrate temperature, usually as apeak metal temperature, of at least about 450° F. Oven air temperaturesmay be more elevated, such as on the order of 650° F. or more. It hasbeen found highly desirable with the hereinbefore describedchromium-free coating compositions to utilize a more elevatedtemperature cure. Such can be on the order of from 330° C. (626° F.) to360° C. (680° F.), with temperatures up to 700° F. being optional. Thus,for these compositions, a peak metal cure temperature range of above650° F. up to about 680° F. or more may be employed. On the other hand,a less elevated peak metal temperature for curing a substrate coatedwith a titanate binder composition of up to about 600° F. can beadvantageous.

[0079] Curing, such as in a hot air convection oven, can be carried onfor several minutes. Although cure times may be less than 5 minutes,they are more typically on the order of from at least about 10 to about45 minutes. It is to be understood that cure times and temperatures canbe effected where more than one layer of coating is applied or whenthere may be a subsequently applied topcoating that is a heat-curedtopcoating. Thus, shorter time and lower temperature cures may beemployed. Also, where more than one coating is applied, or with aheat-curable topcoating, the coating may only need be dried, asdiscussed hereinabove. Then, curing can proceed after application of theheat-cured topcoating.

[0080] The resulting weight of the coating on the metal substrate mayvary to a considerable degree, but, usually excepting for a silicasubstance coating, will generally be present in an amount supplyinggreater than 500 mg/ft² of coating. A lesser amount may generally notlead to desirably enhanced corrosion-resistance. Advantageously, acoating of greater than about 1,000 mg/ft² of coated substrate will bepresent for best corrosion-resistance. It has been found that a coatingweight on the order of about 1,800 mg/ft² can be most advantageous for acoating from the chromium-free coating compositions. It can often beexpected that between about 1,500 to 9,000 mg/ft² of coating will bepresent. Under these general considerations, the particulate metal inthe coating will typically be present in an amount from about 500 mg/ft²to about 5,000 mg/ft².

[0081] Topcoating

[0082] Often, except where otherwise detailed herein, there need not beapplied any topcoating, especially with the above-described compositionsof particular interest. This can be the case when the above-describedcoating compositions are used for a single coating layer, or amulti-coating layer. For example, with the chromium-free coatingcompositions, usually two or three coating layers will be sufficient toachieve a highly desirable corrosion-resistant coating. However, thefollowing discussion is offered where topcoating considerations mayapply.

[0083] Silica Substance Topcoating

[0084] In the present invention, the coated substrate may be topcoated,as with silica substance. The term “silica substance”, as it is usedherein for the topcoating, is intended to have the same meaning as forthe above-described silica substance coating composition, e.g., includesilicates, silicate esters and colloidal silicas. The colloidal silicasinclude both those that are solvent-based as well as aqueous systems,with the water-based colloidal silicas being most advantageous foreconomy. As is typical, such colloidal silicas can include additionalingredients, e.g., thickeners as, for example, up to about 5 weightpercent of an above-discussed water-soluble cellulose ether. Also, aminor amount, e.g., 20 to 40 percent by weight and usually a lesseramount, of the colloidal silicas can be replaced by colloidal alumina.In general, the use of colloidal silicas will provide for heaviertopcoats of silica substance over undercoated substrate materials. It iscontemplated to use colloidal silicas containing up to 50 percent byweight solids, but typically, much more concentrated silicas will bediluted, for example, where spray application of the topcoat will beused.

[0085] When the topcoating silica substance is silicate, it may beorganic or inorganic. The useful organic silicates include the alkylsilicates, e.g., ethyl, propyl, butyl and polyethyl silicates, as wellas alkoxyl silicates such as ethylene glycol monoethyl silicate. Mostgenerally for economy, the organic silicate is ethyl silicate.Advantageously, the inorganic silicates are used for best economy andcorrosion-resistance performance. These are typically employed asaqueous solutions, but solvent-based dispersions may also be used. Whenused herein in reference to silicates, the term “solution” is meant toinclude true solutions and hydrosols. The preferred inorganic silicatesare the aqueous silicates that are the water-soluble silicates,including sodium, potassium, lithium and sodium/lithium combinations, aswell as other related combinations. Referring to sodium silicate asrepresentative, the mole ratios of SiO₂ to Na₂O generally range between1:1 and 4:1. For best efficiency and economy, an aqueous-based sodiumsilicate is preferred as the silica substance. The use of silicasubstance as a topcoating has been described in U.S. Pat. No. 4,365,003,the disclosure of which is incorporated herein by reference.

[0086] Other ingredients may be present in the silica substancetopcoating composition, e.g., wetting agents and colorants, and thecomposition may contain chrome substituents if desired, but can bechrome-free as defined hereinabove to provide a totally chrome-freecoating. Substances that may be present can further include thickeningand dispersing agents as well as pH adjusting agents, but all suchingredients will typically not aggregate more than about 5 weightpercent, and usually less, of the topcoating composition so as toprovide for enhanced coating composition stability coupled withaugmented coating integrity. The silica substance topcoating may beapplied by any of the above described various techniques for use withthe coating composition, such as immersion techniques including dipdrain and dip spin procedures.

[0087] The preferred topcoats are provided from the topcoatingcompositions PLUS®, PLUS® L, PLUS® ML and PLUS® XL made by MetalCoatings International Inc. These may contain inorganic silicatetogether with one or more additional ingredients, e.g., lubricantsincluding wax or polymeric materials, such as polyethylene, copolymersincorporating polyethylene, or polytetrafluoroethylene. Otherconstituents, which may also be used at least in part for theirlubricity, can include graphite and molybdenum disulfide. The topcoatsmay be pigmented, e.g., to provide a black topcoating. A representativeblack topcoating composition has been disclosed in U.S. Pat. No.5,006,597.

[0088] By any coating procedure, the topcoat should be present in anamount above about 50 mg/ft² of coated substrate. For economy, topcoatweights for cured topcoating will not exceed about 2,000 mg/ft² ofcoated substrate. This range is for the cured silica substancetopcoating. Preferably, for best coating efficiency and silica substancetopcoat economy, the topcoat is an inorganic silicate providing fromabout 200 to about 1,200 mg/ft² of cured silicate topcoating.

[0089] For the silica substance topcoat curing, it is typical to selectthe curing conditions in accordance with the particular silica substanceused. For the colloidal silicas, air-drying may be sufficient; but, forefficiency, elevated temperature curing is preferred for all the silicasubstances. The elevated temperature curing can be preceded by drying,such as air-drying. Regardless of prior drying, a lower curetemperature, e.g., on the order of about 150° F. to about 300° F., willbe useful for the colloidal silicas and organic silicates. For theinorganic silicates, curing typically takes place at a temperature onthe order of about 300° F. to about 500° F. In general, curetemperatures on the order of from about 150° F. to about 700°-800° F. ormore, as peak metal temperatures, may be useful. At the more elevatedtemperatures, cure times may be as fast as about 10 minutes, althoughlonger cure times, up to about 20 minutes, are more usual. Also,articles can be topcoated with the silica substance topcoat while thearticles are at elevated temperature, as from the curing of achrome-free coating composition. Such could be done as by spray coat ordip drain, i.e., a dipping of the elevated temperature article into thetopcoat composition, which can provide a quenching of the article. Uponremoval from the topcoating composition, the article can be drained.Some to all of the topcoat curing can be achieved by the operation.

[0090] Electrodeposited Topcoating

[0091] Of special interest, the coated substrate can form a particularlysuitable substrate for paint deposition by electrocoating. Theelectrodeposition of film-forming materials is well known and caninclude electrocoating of simply a film-forming material in a bath orsuch a bath which may contain one or more pigments, metallic particles,drying oils, dyes, extenders, and the like, and the bath may be adispersion or ostensible solution and the like. Some of the well knownresinous materials useful as film-forming materials include thepolyester resins, alkyd resins, acrylate resins, hydrocarbon resins, andepoxy resins, and such materials can be reacted with other organicmonomers and/or polymers including hydrocarbons such as ethylene glycol,monohydric alcohols, ethers, and ketones.

[0092] Also of interest are polycarboxylic acid resins which can besolubilized with polyfunctional amino compounds and include thesiccative oil-modified poly-basic acids, esters or anhydrides which canbe further reacted with divinyl benzene for example or acrylic acid andesters as well as polymerizable vinyl monomers. Further, substances ofinterest are the anodically deposited film-forming materials. However,the broad scope to which the electrodeposition of film-forming materialsrelates, includes the deposition of such materials on anodic or cathodicsubstrates, and by means of various techniques for passage of currentthrough a bath. After electrodeposition and removal of the coatedsubstrate from the bath, curing of the film-forming materials can beperformed. The time and temperature of curing will be dependent upon thefilm-forming materials present, but is typically an air cure at roomtemperature or a forced cure at a temperature up to 500° F. and fortimes up to 60 minutes, at more reduced temperatures.

[0093] Quench Coat Topcoating

[0094] An additional topcoat of special interest is a coating applied byquench coating. Thus the coated substrate may proceed to a quenchcoating, e.g., following heat curing of an above-described chromium-freecoating composition, or even following cure of a topcoating such as atopcoating of silica substance. Such quench coating of articles atelevated temperature by bringing them into contact with an aqueous resinsolution has been discussed in Japanese Patent Application No. 53-14746.Suitable resin solutions include alkyd, epoxy, melamine and urea resins.

[0095] For this, it has also been taught, for example in U.S. Pat. No.4,555,445, that suitable topcoating compositions may be pigmenteddispersions or emulsions. These can include copolymer dispersions inliquid medium as well as aqueous emulsions and dispersions of suitablewaxes. Articles can be topcoated in these compositions, which articlesare at elevated temperature such as after curing of the appliedwater-reducible coating, by procedures including a dip-drain or a spraycoating operation. By such quench coating operation, all of thetopcoating curing may be achieved without further heating. Quenchcoating with polymeric solutions, emulsions and dispersions, and withheated baths, has also been discussed in U.S. Pat. No. 5,283,280.

[0096] Autodeposited Topcoating

[0097] Another topcoat of particular interest is the autodepositedcoating. The autodeposition of coatings provides a latex-based coatingfilm on metal articles, with no external voltage applied in the process.In the U.S. Pat. No. 3,592,699, it is taught to apply a coating from abath of suitable polymer latex, oxidizing agent, fluoride ion andsufficient acid to maintain a pH of from about 2.5 to 3.5. Formulated assuch an acidic composition, the bath can use metal dissolution as adriving force for coating deposition. More recently, U.S. Pat. No.5,300,323 has taught a zinc surface pretreatment with an aqueoushydrogen fluoride solution containing an additive such as boric acid.This can help negate the formation of pinholes during autodepositioncoating.

[0098] Further Topcoating

[0099] The coated substrate may also have a topcoat with any othersuitable topcoating, i.e., a paint or primer, including weldableprimers, such as the zinc-rich primers that may be typically appliedbefore electrical-resistance welding. For example, it has already beenshown in U.S. Pat. No. 3,671,331 that a primer topcoating containing aparticulate, electrically conductive pigment, such as zinc, is highlyserviceable for a metal substrate that is first coated with anothercoating composition. Other topcoating paints may contain pigment in abinder or can be unpigmented, e.g., generally cellulose lacquers, resinvarnishes, and oleoresinous varnishes, as for example tung oil varnish.The paints can be solvent-reduced or they may be water-reduced, e.g.,latex or water-soluble resins, including modified or soluble alkyds, orthe paints can have reactive solvents such as in the polyesters orpolyurethanes. Additional suitable paints which can be used include oilpaints, including phenolic resin paints, solvent-reduced alkyds,epoxies, acrylics, vinyl, including polyvinyl butyral, and oil-wax-typecoatings such as linseed oil-paraffin wax paints.

[0100] Before any coating, it is in most cases advisable to removeforeign matter from the substrate surface, as by thoroughly cleaning anddegreasing. Degreasing may be accomplished with known agents, forinstance, with agents containing sodium metasilicate, caustic soda,carbon tetrachloride, trichlorethylene, and the like. Commercialalkaline cleaning compositions, which combine washing and mild abrasivetreatments, can be employed for cleaning, e.g., an aqueous trisodiumphosphate-sodium hydroxide cleaning solution. In addition to cleaning,the substrate may undergo cleaning plus etching, or cleaning plus shotblasting.

[0101] The following examples show ways in which the invention has beenpracticed but should not be construed as limiting the invention. In theexamples, the following procedures have been employed:

[0102] Preparation of Test Panels

[0103] Unless otherwise specifically described, test panels are coldrolled, low carbon steel panels. Steel panels can be prepared forcoating by first immersing in a cleaning solution. A metal cleaningsolution can contain 5 ounces, per each gallon of water, of a mixture of25 weight percent tripotassium phosphate and 75 weight percent potassiumhydroxide. This alkaline bath is maintained at a temperature of about150° F. to 180° F. Following solution cleaning, the panels can bescrubbed with a cleaning pad, which is a porous, fibrous pad ofsynthetic fiber impregnated with an abrasive. Thereafter, the scrubbedpanels are water-rinsed and again immersed in cleaning solution.Following removal from the solution, the panels are rinsed with tapwater and preferably dried.

[0104] Application of Coating to Test Parts and Coating Weight

[0105] Unless otherwise described in the example, clean parts aretypically coated by dipping into coating composition, removing anddraining composition therefrom, spinning off the excess, and thenimmediately baking or air drying at room temperature or precuring atmodest temperature until the coating is dry to the touch and thenbaking. Baking and precuring proceeds in a hot air convection oven attemperatures and with times as specified in the examples.

[0106] Coating weights for panels, generally expressed as a weight perunit of surface area, is typically determined by selecting a panel of aknown surface area and weighing it before coating. After the panel hasbeen coated, it is reweighed and the coating weight per selected unit ofsurface area, most always presented as milligrams per square foot(mg/ft²), is arrived at by straightforward calculation.

[0107] Coating Adhesion Test

[0108] This test is conducted by manually pressing a strip of tapecoated with a pressure-sensitive adhesive against the coated surface ofthe test panel, which tape is then quickly removed. The coating isevaluated qualitatively according to the amount of coating removed bythe adhesive on the tape, in comparison with the condition of a standardtest panel.

[0109] Corrosion-Resistance Test (ASTM B117) and Rating

[0110] Corrosion-resistance of coated parts is measured by means of thestandard salt spray (fog) test for paints and varnishes ASTM B-117. Inthis test, the parts are placed in a chamber kept at constanttemperature where they are exposed to a fine spray (fog) of a 5 percentsalt solution for specified periods of time, rinsed in water and dried.The extent of corrosion of the test parts can be expressed as percent ofred rust.

EXAMPLE 1

[0111] To 18.9 weight parts of deionized water, there is blended withmoderate agitation, 0.6 weight part of ortho boric acid and 3 weightparts of gamma-glycidoxypropyltrimethoxysilane as blending continues.After mixing continues for 3 hours, there is added to this mixture anadditional 31 weight parts of deionized water and a wetter blendcontaining 0.8 weight part of a nonionic, ethoxylated nonylphenol wetter(“nenw”) having a molecular weight of 396 and a specific gravity of1.0298 at 20/20° C. and 0.8 weight part of a “nenw” having a molecularweight of 616 and a specific gravity of 1.057 at 20/20° C. To thismixture there is then added an additional 2 weight parts of theabove-mentioned silane, 2.2 weight parts of dipropylene glycol and 0.7weight part of 1-nitropropane. To this mixture there is added 35.2weight parts of STAPA 4ZnAl7 zinc and aluminum alloy flake paste. Thepaste contains about 85 weight percent zinc, about 5 weight percentaluminum, and a balance of paste liquid. The alloy flake has about 50percent of the flake particles with a longest dimension of discreteparticles of less than about 13 microns. The sum of all of theseingredients are then ground for about 3 hours using a Cowles dissolveroperating at approximately 800 revolutions per minute (rpm).

[0112] To the resulting ground mixture, there is then added, whilestirring is continued for 1 hour, 0.4 weight part of sodium bistridecylsulfosuccinate anionic surfactant and mixing is then further continuedovernight. There is then added 2.9 weight parts of additional,above-described silane plus a slurry of 0.2 weight part of hydroxy ethylcellulose slurried in 1.3 weight parts of deionized water. This bath isaged for 6 days. This resulting coating composition had a molar ratio ofwater to silane alkoxy groups of 30.5:1.

[0113] A clean 3×5-inch test panel as described hereinabove was thencoated by drawing the coating composition down over the panel with adraw bar. The panel is precured for 10 minutes at an oven airtemperature of 150° F. and cured for 30 minutes at an oven airtemperature of 600° F., all in the manner as described hereinabove. Theresulting panel had a smooth, grey coating of attractive appearance. Thecoating weight was 1968 milligrams per square foot of coated panelsurface and the coating had acceptable coating adhesion. Coating weightand coating adhesion were determined as described hereinabove.

[0114] Bolts were prepared for coating as described hereinabove, exceptno scrubbing is used during cleaning and the bolts are cleaned byblasting with glass beads (dry honed) after oven drying. The bolts arecoated by placing in a wire basket and dipping the basket into thecoating composition, removing the basket and draining excess compositiontherefrom. During dip spinning, for the first coat, the basket is spunat 275 rpm for 10 seconds forward and 10 seconds reverse and, for thesecond coat, at 300 rpm and again at 10 seconds forward and 10 secondsreverse.

[0115] Draining is then followed by baking. The bolts are usually placedon a sheet for baking. Baking proceeds first at an air temperature ofabout 150° F. for a time up to 10 minutes and then at 600° F. for 30minutes. The bolts are coated twice with the coating composition usingthis procedure and providing a coating weight of 3,138 mg/ft²,determined as described hereinabove.

[0116] Selected bolts are then topcoated with a commercially availablesodium silicate topcoat composition disclosed in the U.S. Pat. No.4,365,003. The procedure used for coating and baking was as for theundercoating, but the basket spin was at 400 rpm for 10 seconds forward,10 seconds reverse, and the cure was at 350° F. for 20 minutes. Coatingweight determinations, conducted in the manner as described above inconnection with the examples, showed a topcoating weight for arepresentative bath of 520 mg/ft².

[0117] The hex-head bolts used in the test are a specific grade of 9.8bolts, which more particularly are 1½ inches long by about {fraction(5/16)} inch in diameter at the threaded end and have 1{fraction (3/16)}inches of threading on the shaft that terminates in the bolt head.

[0118] The resulting coated bolts were then subjected to thehereinbefore described corrosion-resistance test. The test bolts withthe silicate topcoat have gone over 2,000 hours in testing without theappearance of first red rust, compared to red rust appearance at 600hours for bolts coated with the same procedure but using a basecoatcomposition containing a simple mixture of zinc flake and aluminum flakein a 90/10 weight ratio.

EXAMPLE 2

[0119] For test purposes, there is prepared a coating composition ofzinc plus tin alloy flake. This preparation is initiated as follows: To18.9 weight parts of deionized water, there is blended with moderateagitation, 0.6 weight part of ortho boric acid and 3 weight parts theExample 1 silane as blending continues to prepare an initial silaneblend. After mixing continues for 3 hours, there is added to thismixture an additional 34 weight parts of deionized water and a blendcontaining 0.8 weight part of the Example 1 wetter having a molecularweight of 396,1.6 weight parts of the Example 1 wetter having amolecular weight of 616, an additional 2 weight parts of theabove-mentioned silane, and 0.7 weight part of 1-nitropropane. To thismixture there is added 32.6 weight parts of STAPA 4ZnSn30 zinc and tinalloy flake paste. The paste contains about 70 weight percent zinc andabout 30 weight percent tin in the alloy flake, on a metals basis, and abalance of paste liquid. The sum of all of these ingredients are thenground for about 3 hours using a Cowles dissolver operating atapproximately 800 revolutions per minute (rpm).

[0120] To the resulting ground mixture, there is then added, whilestirring is continued for 1 hour, 0.4 weight part of sodium bistridecylsulfosuccinate anionic surfactant and mixing is then further continuedovernight. There is then added 2.9 weight parts of additional,above-described silane plus a slurry of 0.3 weight part of hydroxy ethylcellulose slurried in 2 weight parts of deionized water.

[0121] For comparative purposes, there is prepared a standardcomparative coating composition using the procedure describedhereinabove. For this composition there is blended with the initialsilane blend an additional 29.4 weight parts of water and a blend of 1.5weight parts of each wetter, 2 weight parts of the silane, 0.7 weightpart of the 1-nitropane, 1.2 weight parts of dipropylene glycol, and 4.3parts of aluminum flake paste. To this there is added 31.2 parts of zincflake paste. Consistent with the above-described procedures and amounts,there is then added the anionic surfactant, additional silane,cellulose/water slurry, and also 0.2 weight part of a liquid blend ofpetroleum derivatives having a specific gravity of 0.9 used as adefoamer.

[0122] In this test, the test parts were M-8 bolts, which are moreparticularly described in Example 3. The bolts were prepared forcoating, and coated, all as described in Example 3, except the curetemperature was 232° C. The resulting parts have a coating weight of2,463 mg/ft². Coating weight was determined by the weigh-coat-weighprocedure as described hereinabove.

[0123] The parts were then topcoated with a commercially availablesodium silicate topcoat composition disclosed in the U.S. Pat. No.4,365,003. The procedure used for coating and baking was as for theundercoating, but the cure was at 176° C. for 20 minutes. Coating weightdetermination, conducted in the manner as described above, showed atopcoating weight of 433 mg/ft².

[0124] The resulting coated parts were then subjected to thehereinbefore described corrosion-resistance test. The test parts withthe silicate topcoat and zinc plus tin alloy undercoat went for 1080hours in testing with a rating of 4.8 on a scale of 5.0 (best) in regardto appearance of red rust. Comparatively the test parts with thesilicate topcoat and the zinc flake plus aluminum flake undercoat at1,080 hours of testing had a rating of 4.2.

EXAMPLE 3

[0125] An invention test composition is prepared with the followingingredients. There is mixed together 7.37 weight parts of the Example 1silane, a wetter blend containing 1.21 weight parts of the Example 1wetter having a molecular weight of 396, and 1.39 weight parts of theExample 1 wetter having a molecular weight of 616, 4.33 weight parts ofdipropylene glycol, 0.62 weight part of 1-nitropropane and 0.45 weightpart of sodium bistridecyl sulfosuccinate anionic surfactant. To thismixture there is added 29.83 weight parts of zinc and aluminum alloyflake paste. The paste contains about 85 weight percent zinc and about 6weight percent aluminum in the alloy flake and an about 9 weight percentbalance paste liquid. The alloy flake has about 98 percent of the flakeparticles with a longest dimension of discrete particles of less thanabout 15 microns. The sum of all of these ingredients is then vigorouslymixed together.

[0126] To the resulting ground mixture, there is then added, whilestirring is continued, a blend of 0.53 weight part boric acid in 54.27weight parts deionized water. There is then added 0.4 weight part ofhydroxy ethyl cellulose and stirring is continued overnight.

[0127] For comparative purposes, there is then used the standardcomparative coating composition described in Example 2.

[0128] In this test, bolts, as more specifically described hereinbelow,are used. The bolts are prepared for coating as described hereinabove,except no scrubbing is used during cleaning and the bolts are cleaned byblasting with glass beads (dry honed) after oven drying. The bolts arecoated by placing in a wire basket and dipping the basket into theoating composition, removing the basket and draining excess compositiontherefrom. After draining, the basket is spun at 300 rpm for 10 secondsforward and 10 seconds reverse.

[0129] Draining is then followed by baking. The bolts are usually placedon a screen for baking. Baking proceeds first at an air temperature ofabout 66° C. for a time up to 10 minutes and then at 329° C. for 30minutes. The bolts are coated twice with the coating composition usingthis procedure and providing a coating weight of about 1,900 mg/ft²,determined as described hereinabove.

[0130] The bolts used in the test are M-8 bolts, which more particularlyare 1.4 inches long by about {fraction (5/16)} inch in diameter at thethreaded end and have 1{fraction (3/16)} inches of threading on theshaft that terminates at the bolt head.

[0131] The resulting coated bolts are then subjected to the Society ofAutomotive Engineers corrosion test SAE J2334. The test cycle was a24-hour test cycle. In each test cycle there was used a humid stage, asalt application stage and a dry stage. The humid stage was 100%humidity for 6 hours at 50° C. The salt application stage was for 15minutes at ambient conditions. The dry stage was 50% humidity at 60° C.for 17 hours and 45 minutes.

[0132] In the test, the bolts coated with the standard comparativecoating showed first red rust at 56 cycles. However, the bolts coatedwith the invention test composition passed 89 cycles with no red rust.

1. In a coating composition adapted for application to, and curing on, asubstrate, which composition contains particulate metal in a liquidmedium and provides corrosion resistance as a cured coating on saidsubstrate, the improvement in the particulate metal constituency of saidcomposition comprising: zinc alloy in flake form comprising greater than50 weight percent zinc in said alloy flake and a balance of less than 50weight percent of non-zinc alloy metal in said alloy flake.
 2. Thecoating composition of claim 1 wherein said zinc alloy in flake form iszinc alloyed with one or more of aluminum, tin, magnesium, nickel,cobalt and manganese.
 3. The coating composition of claim 1 wherein saidzinc is alloyed with one or more of tin and aluminum, with said zincalloyed with aluminum containing less than about 20 weight percentaluminum, while said zinc alloyed with tin contains not more than about30 weight percent tin.
 4. The coating composition of claim 1 whereinsaid zinc alloy in flake form is a zinc-aluminum-magnesium alloy flake.5. The coating composition of claim 1 wherein said zinc alloy in flakeform comprises a paste containing less than about 15 weight percentaluminum in said alloy flake, on a metals basis, and up to about 10weight percent paste liquid, basis weight of said paste.
 6. The coatingcomposition of claim 5 wherein said paste contains from about 85 toabout 86 weight percent zinc in said alloy and from about 4 to about 8weight 30 percent of aluminum in said alloy, both basis 100 weightpercent of said paste.
 7. The coating composition of claim 5 whereinsaid paste contains from about 7 to about 10 weight percent of pasteliquid and contains from about 4 to about 5 weight percent of saidaluminum, both basis 100 weight percent of said paste.
 8. The coatingcomposition of claim 6 wherein said paste is STAPA 4ZnAl7.
 9. Thecoating composition of claim 1 wherein said zinc alloy in flake form isan alloy having at least about 90 percent of the flake particles with alongest dimension of less than about 15 microns and has at least about50 percent of the flake particle with a longest dimension of less thanabout 13 microns, and said composition further contains non-alloyedparticulate metal.
 10. The method of preparing a corrosion-resistantcoated substrate protected with a corrosion-resistant coating, whichmethod comprises: (1) applying to said substrate a coating compositioncomprising: (A) liquid medium; and (B) zinc alloy in flake formcomprising greater than 50 weight percent zinc in said alloy flake and abalance of less than 50 weight percent of non-zinc alloy metal; and (2)curing applied coating composition on said substrate.
 11. The method ofclaim 10 wherein there is applied a coating composition comprising acombination of a liquid medium plus a zinc alloy in flake form, whichcombination is a paste containing at least about 70 weight percent zincin said alloy flake, on a metals basis, and up to about 10 weightpercent paste liquid basis weight of said paste.
 12. A coated substrateprotected with a chrome-free, corrosion-resistant coating from acomposition comprising: (A) liquid medium; (B) zinc alloy in flake formcomprising greater than 50 weight percent zinc in said alloy flake and abalance of less than 50 weight percent of non-zinc alloy metal; and (C)silane binding agent.
 13. The coated substrate of claim 12 wherein saidliquid medium is one or more of water and organic liquid and said water,when present, is present in an amount above about 25 weight percent ofsaid coating composition.
 14. The coated substrate of claim 12 whereinsaid zinc alloy in flake form is zinc alloyed with one or more ofaluminum, magnesium, tin, nickel, cobalt and manganese.
 15. The coatedsubstrate of claim 12 wherein said zinc alloy in flake form is azinc-aluminum-magnesium alloy flake.
 16. The coated substrate of claim12 wherein said zinc alloy in flake form is an alloy having at leastabout 90 percent of the flake particles with a longest dimension of lessthan about 15 microns and has at least about 50 percent of the flakeparticle with a longest dimension of less than about 13 microns, andsaid composition further contains non-alloyed particulate metal.
 17. Thecoating composition of claim 12 wherein said zinc is alloyed with one ormore of tin and aluminum, with said zinc alloyed with aluminumcontaining less than about 20 weight percent aluminum, while said zincalloyed with tin contains not more than about 30 weight percent tin. 18.The coated substrate of claim 12 wherein said zinc alloy in flake formcomprises a paste containing less than about 15 weight percent aluminumin said alloy flake, on a metals basis, and up to a bout 10 weightpercent paste liquid, basis weight of said paste.
 19. The coatedsubstrate of claim 18 wherein said paste contains from about 85 to about86 weight percent zinc in said alloy and from about 4 to about 8 weightpercent of aluminum in said alloy, both basis 100 weight percent of saidpaste.
 20. The coated substrate of claim 18 wherein said paste containsfrom about 7 to about 10 weight percent of paste liquid and containsfrom about 4 to about 5 weight percent of said aluminum, both basis 100weight percent of said paste.
 21. The coated substrate of claim 19wherein said zinc-aluminum alloy paste is STAPA 4ZnAl7.
 22. The coatedsubstrate of claim 12 wherein said silane binding agent is awater-reducible, organofunctional binding agent containing alkoxygroups, which silane binding agent contributes from about 3 to about 20weight percent of said coating composition.
 23. The coated substrate ofclaim 12 wherein said coating composition has a pH within the range offrom greater than 6 to about 7.5, contains water in an amount aboveabout 30 weight percent, and has a molar ratio of water to silane alkoxygroups above about 4.5:1.
 24. The coated substrate of claim 12 whereinsaid coating additionally contains one or more of thickener and wettingagent, said coating is topcoated with a composition containing silicasubstance and said topcoating provides silica substance from one or moreof colloidal silica, organic silicate and inorganic silicate.
 25. Themethod of preparing a corrosion-resistant coated substrate protectedwith a chrome-free, corrosion-resistant coating, which method comprises:(1) applying to said substrate a coating composition comprising: (A)liquid medium; (B) zinc alloy in flake form comprising greater than 50weight percent zinc in said alloy flake and a balance of less than 50weight percent of non-zinc alloy metal; and (C) silane binding agent;with said coating composition being applied in an amount sufficient toprovide, upon curing, above about 500 but not substantially above about9,000 mg/ft² of coating on said metal substrate; and (2) heat curingapplied coating composition on said substrate at a temperature up toabout 700° F. for a time of at least about 10 minutes.
 26. The method ofclaim 25 wherein said coating composition has a zinc alloy pastecomprising at least about 70 weight percent zinc in said alloy flake, upto about 10 weight percent paste liquid, and a balance of additionalalloy metals and said composition is applied in an amount sufficient toprovide, upon curing, above about 1,500 mg/ft² of coating on said coatedsubstrate.
 27. The method of claim 25 wherein said applied coatingcomposition is cured at an elevated temperature within the range fromabout 330° C. (626° F.) to about 360° C. (680° F.).
 28. A coatedsubstrate protected with a corrosion-resistant coating from a coatingcomposition comprising: (A) liquid medium; (B) zinc alloy in flake formcomprising greater than 50 weight percent zinc in said alloy flake and abalance of less than 50 weight percent of non-zinc alloy metal; and (C)a hexavalent-chromium-providing substance.
 29. The coated substrate ofclaim 28 wherein said liquid medium is one or more of water and organicliquid.
 30. The coated substrate of claim 28 wherein said zinc alloy inflake form is zinc alloyed with one or more of aluminum, tin, magnesium,nickel, cobalt and manganese.
 31. The coated substrate of claim 28wherein said alloy flake is a zinc-aluminum-magnesium alloy flake. 32.The coated substrate of claim 28 wherein said zinc alloy in flake formis an alloy having at least about 90 percent of the flake particles witha longest dimension of less than about 15 microns and has at least about50 percent of the flake particle with a longest dimension of less thanabout 13 microns, and said composition further contains non-alloyedparticulate metal.
 33. The coating composition of claim 28 wherein saidzinc is alloyed with one or more of tin and aluminum, with said zincalloyed with aluminum containing less than about 20 weight percentaluminum, while said zinc alloyed with tin contains not more than about30 weight percent tin.
 34. The coated substrate of claim 28 wherein saidzinc alloy in flake form comprises a paste containing less than about 15weight percent aluminum in said alloy flake, on a metals basis, and upto about 10 weight percent paste liquid, basis weight of said paste. 35.The coated substrate of claim 28 wherein said paste contains from about85 to about 86 weight percent zinc in said alloy and from about 4 toabout 8 weight percent of aluminum in said alloy, both basis 100 weightpercent of said paste.
 36. The coated substrate of claim 35 wherein saidpaste contains from about 7 to about 10 weight percent of paste liquidand contains from about 4 to about 5 weight percent of said aluminum,both basis 100 weight percent of said paste.
 37. The coated substrate ofclaim 35 wherein said zinc-aluminum alloy paste is STAPA 4ZnAl7.
 38. Thecoated substrate of claim 28 wherein said coating additionally containsone or more of thickener and wetting agent, said coating is topcoatedwith a composition containing silica substance, and said topcoatingprovides silica substance from one or more of colloidal silica, organicsilicate and inorganic silicate.
 39. The method of preparing acorrosion-resistant coated substrate protected with a chrome-free,corrosion-resistant coating, which method comprises: (1) applying acoating composition comprising (A) liquid medium; (B) zinc alloy inflake form comprising greater than 50 weight percent zinc in said alloyflake and a balance of less than 50 weight percent of non-zinc alloymetal; and (C) a hexavalent-chromium-providing substance; with saidcoating composition being applied in an amount sufficient to provide,upon curing, above about 500 but not substantially above about 9,000mg/ft² of coating on said coated substrate; and, (2) heat curing appliedcoating composition on said substrate at a temperature up to about 700°F. for a time of at least about 10 minutes.
 40. The method of claim 39wherein said coating composition has a zinc alloy paste comprising atleast about 70 weight percent zinc in said alloy flake, up to about 10weight percent paste liquid, and a balance of additional alloy metalsand said composition is applied in an amount sufficient to provide, uponcuring, above about 1,800 mg/ft² of coating on said coated substrate.41. The method of claim 39 wherein said applied coating composition iscured at an elevated temperature within the range from about 330° C.(626° F.) to about 360° C. (680° F.).
 42. A coated substrate protectedwith a chrome-free, corrosion-resistant coating from a coatingcomposition comprising: (A) zinc alloy in flake form comprising greaterthan 50 weight percent zinc in said alloy flake and a balance of lessthan 50 weight percent of non-zinc alloy metal; (B) a titanate polymer;and (C) a liquid vehicle comprising organic liquid for said titanatepolymer.
 43. The coated substrate of claim 42 wherein said coatingcomposition additionally contains manganese dioxide, and said manganesedioxide is present in an amount equal to about 30 weight percent toabout 100 weight percent of said zinc alloy in flake form.
 44. Thecoated substrate of claim 42 wherein said liquid vehicle is a blend ofwater with organic liquid.
 45. The coated substrate of claim 42 whereinsaid zinc alloy in flake form is zinc alloyed with one or more ofaluminum, tin, magnesium, nickel, cobalt and manganese.
 46. The coatedsubstrate of claim 42 wherein said zinc alloy in flake form is azinc-aluminum-magnesium alloy flake.
 47. The coated substrate of claim42 wherein said zinc alloy in flake form is an alloy having at leastabout 90 percent of the flake particles with a longest dimension of lessthan about 15 microns and has at least about 50 percent of the flakeparticle with a longest dimension of less than about 13 microns, andsaid composition further contains non-alloyed particulate metal.
 48. Thecoating composition of claim 42 wherein said zinc is alloyed with one ormore of tin and aluminum, with said zinc alloyed with aluminumcontaining less than about 20 weight percent aluminum, while said zincalloyed with tin contains not more than about 30 weight percent tin. 49.The coated substrate of claim 42 wherein said zinc alloy in flake formcomprises a paste containing less than about 15 weight percent aluminumin said alloy flake, on a metals basis, and up to about 10 weightpercent paste liquid, basis weight of said paste.
 50. The coatedsubstrate of claim 49 wherein said paste contains from about 85 to about86 weight percent zinc in said alloy and from about 4 to about 8 weightpercent of aluminum in said alloy, both basis 100 weight percent of saidpaste.
 51. The coated substrate of claim 50 wherein said paste containsfrom about 7 to about 10 weight percent of paste liquid and containsfrom about 4 to about 5 weight percent of said aluminum, both basis 100weight percent of said paste.
 52. The coated substrate of claim 50wherein said zinc-aluminum alloy paste is STAPA 4ZnAl7.
 53. The coatedsubstrate of claim 42 wherein said titanate polymer is selected from thegroup consisting of tetraisobutyl titanate, tetra-isopropyl titanate,tetra N-butyl titanate and mixtures thereof, and said titanate ispresent in an amount equal to about 9 weight percent to about 47 weightpercent of said metal alloy in flake form.
 54. The coated substrate ofclaim 42 wherein said coating is topcoated.
 55. The method of preparinga corrosion-resistant coated substrate protected with a chrome-free,corrosion-resistant coating, which method comprises: (1) applying acoating composition comprising: (A) zinc alloy in flake form comprisinggreater than 50 weight percent zinc in said alloy flake and a balance ofless than 50 weight percent of non-zinc alloy metal; (B) a titanatepolymer; and (C) a liquid vehicle comprising organic liquid for saidtitanate polymer; and (2) heat curing applied coating composition onsaid substrate at a temperature up to about 600° F. for a time of atleast about 10 minutes.
 56. The method of claim 55 wherein said coatingcomposition has a zinc alloy paste comprising at least about 70 weightpercent zinc in said alloy flake, up to about 10 weight percent pasteliquid, and a balance of additional alloy metals and said composition isapplied in an amount sufficient to provide, upon curing, above about1,800 mg/ft² of coating on said coated substrate.
 57. A coated substrateprotected with a corrosion-resistant coating from a coating compositioncomprising: (A) liquid medium; (B) zinc alloy in flake form comprisinggreater than 50 weight percent zinc in said alloy flake and a balance ofless than 50 weight percent of non-zinc alloy metal; and (C) one or moreof a water-soluble and water dispersible silica substance.
 58. Thecoated substrate of claim 57 wherein said silica substance is selectedfrom the group consisting of alkali metal silicate, organic silicateester, colloidal silica sol, organic ammonium silicate and mixtures ofthe foregoing.
 59. The coated substrate of claim 57 wherein saidcomposition has a water-based liquid medium and additionally containsone or more of a thickening agent and metallic oxide pigment.
 60. Thecoated substrate of claim 59 wherein said thickening agent is one ormore of cellulose ether and xanthan gum and said metallic oxide pigmentis one or more of zinc oxide, iron oxide and titanium oxide.
 61. Thecoated substrate of claim 57 wherein said zinc alloy in flake form iszinc alloyed with one or more of aluminum, tin, magnesium, nickel,cobalt and manganese.
 62. The coated substrate of claim 57 wherein saidzinc alloy in flake form is a zinc-aluminum-magnesium alloy flake. 63.The coated substrate of claim 57 wherein said zinc alloy in flake formis an alloy having at least about 90 percent of the flake particles witha longest dimension of less than about 15 microns and has at least about50 percent of the flake particle with a longest dimension of less thanabout 13 microns, and said composition further contains non-alloyedparticulate metal.
 64. The coating composition of claim 57 wherein saidzinc is alloyed with one or more of tin and aluminum, with said zincalloyed with aluminum containing less than about 20 weight percentaluminum, while said zinc alloyed with tin contains not more than about30 weight percent tin.
 65. The coated substrate of claim 57 wherein saidzinc alloy in flake form comprises a paste containing less than about 15weight percent aluminum in said alloy flake, on a metals basis, and upto about 10 weight percent paste liquid, basis weight of said paste. 66.The coated substrate of claim 65 wherein said paste contains from about85 to about 86 weight percent zinc in said alloy and from about 4 toabout 8 weight percent of aluminum in said alloy, both basis 100 weightpercent of said paste.
 67. The coated substrate of claim 66 wherein saidpaste contains from about 7 to about 10 weight percent of paste liquidand contains from about 4 to about 5 weight percent of said aluminum,both basis 100 weight percent of said paste.
 68. The coated substrate ofclaim 66 wherein said zinc-aluminum alloy paste is STAPA 4ZnAl7.
 69. Thecoated substrate of claim 65 wherein said coating is topcoated.
 70. Themethod of preparing a coated substrate protected with acorrosion-resistant coating, which method comprises: (1) applying acoating composition comprising: (A) liquid medium; (B) zinc alloy inflake form comprising greater than 50 weight percent zinc in said alloyflake and a balance of less than 50 weight percent of non-zinc alloymetal; and (C) one or more of a water-soluble and water dispersiblesilica substance; and (2) heat curing applied coating composition onsaid substrate at a temperature up to about 700° F. for a time of atleast about 10 minutes.
 71. The method of claim 70 wherein said coatingcomposition has a zinc alloy paste comprising at least about 70 weightpercent zinc in said alloy flake, up to about 10 weight percent pasteliquid, and a balance of additional alloy metals and said composition isapplied in an amount sufficient to provide, upon curing, above about1,800 mg/ft² of coating on said coated substrate.